(1) Field of the Invention
The invention relates to an electric battery that is intended in particular for the front-wheel drive of an electric motor vehicle or a hybrid motor vehicle, i.e. including an electric engine powering the drive wheels combined with a thermal engine powering the same or possibly other drive wheels.
(2) Prior Art
To ensure the required power and energy levels for electric or hybrid vehicle applications, it is necessary to create batteries including a plurality of electric energy generating elements.
When these elements are charged and discharged, heat is produced, which, when it is not controlled, can decrease the lifetime of the elements, and even, under extreme conditions, present risks of thermal runaway for certain chemical compositions of elements, leading to deterioration of the battery.
The energy that a battery is capable of providing is dependent on the energy balance of the various elements as well as their operating temperatures. Indeed, the energy that can be generated by an element increases with the temperature, and when there are differences in energy levels available in each of the elements, for a single battery, then the battery is said to be unbalanced. This imbalance strongly affects the performance of the battery in terms of both lifetime and average energy density, because the total energy that can be generated by a battery is always limited by the energy of the element with the lowest charge, and the total energy is moreover limited by the element with the highest charge.
These differences in energy level between the elements, causing the imbalance, can be due either to differences between the electric properties of the elements or to variations in the operating temperatures of the elements. When one element of a battery has a lower charge than the others, a risk of inversion may appear for low charge states.
Furthermore, the chemical compositions of lithium-ion batteries are more or less stable. When they are activated under extreme conditions, thermal runaway can appear. For large batteries that are necessary for predominantly electric vehicles, this risk is critical, because if the thermal runaway of an element spreads to the entire battery, the energy involved by this runaway becomes very high.
To optimize the performance and lifetime of batteries, systems for thermal conditioning of elements have therefore been integrated in the batteries.
In particular, cooling systems have been proposed, which use air circulation as a heat sink. While numerous efforts have been made to ensure the most uniform temperature possible by these means within the battery, it remains that such system do not uniformly cool the power-activated battery elements, as is the case in particular in applications intended for plug-in electric and hybrid vehicles.
The thermal dissipation peaks are very high and are dependent on the current densities and the variations thereof, which, for specific applications, can reach very high values, in particular during phases of high accelerations, regenerative brakings, rapid recharging of the battery, or highway operation in electric mode.
For such conditions of use, the air flow rates necessary to cool the battery elements can be reached only by providing significant space between the elements.
These high flow rates serve to compensate for the low heat transfer coefficients of the air flows over the battery elements, and lead to acoustic and vibration problems. The fans necessary for ensuring the flow rates enabling the batteries to be cooled uniformly and effectively then have sizes that are unsuitable for the requirements of compactness and energy savings of the electric vehicle application.
To improve the efficacy of the cooling, and simultaneously enable the volume energy density of the batteries to be increased, the circulation of a liquid has been proposed. In particular, the liquid can be provided in order to circulate through plastic cells that are arranged between the battery elements. These cells are insulating and contribute to the electrical insulation between elements.
However, the plastic pouches in which these cells are formed are poor thermal conductors, so they must have the lowest thickness possible to ensure the proper heat transfers. This results in an unsuitability of the fine walls for the mechanical strength of the battery elements.
Moreover, in the electric or hybrid vehicle application, the batteries according to the prior art present a certain number of problems, in particular due to the increase in the degree of hybridization of thermal vehicles, which can be a complete electrification of the front-wheel drive chain. In this case, the batteries no longer serve solely to assist the vehicles in acceleration phases, but also ensure the autonomous movement of the vehicle over substantial distances.
It is then necessary to increase the electric power of the batteries, which increases the battery activation times, as well as the currents and the average internal resistance. Thus, the energy and the thermal power emitted increase, and all the more so as the battery ages.
The cost of a battery is dependent primarily on the number of elements that it contains, or in other words, on its energy. Also, to reduce the impact of the cost of batteries in a vehicle, it is sought to use said batteries over the broadest possible potential range so as to extract the maximum amount of energy therefrom.
As one gets closer to the extreme potential values allowed, the internal resistance of the elements increases, and their lifetimes decrease.
The high powers required lead to significant and rapid heating of the battery elements, which can cause temperature gradients between the surface and the interior thereof, and even between elements in the same battery.
These temperature gradients essentially appear during the transient phases corresponding to high current surges, during charging or discharging.
The increase in temperature within a battery element leads to risks in terms of safety and lifetime, associated with the possible presence of hot spots at the core of the element.
Battery safety is also becoming more critical with the increase in energy of batteries, and the plastic cells generally used to circulate a coolant between the elements are capable of breaking under impacts such as those that occur in a vehicle crash, or by overpressure generated at the level of the cooling circuit.
Such breakages then make the cooling system entirely inoperative, but even worse, the liquid is capable of causing a short circuit of all of the battery elements, thus creating a real risk of fire, and even explosion.